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CHICAGO — A new study shows that children’s birthday celebrations could be partly to blame for the deadly rise of COVID-19 cases in early 2020.

Researchers looked at anonymous health insurance data of 2.9 million households from the first 45 weeks of the year to see if such gatherings were linked with higher rates of COVID-19 transmission.

In counties with higher rates of coronavirus overall, households that had a birthday during that time saw a 31% relative increase in COVID diagnoses than households without a birthday, according to the study, which was published in JAMA Internal Medicine.

In those counties the average jumped by 8.6 cases per 10,000 individuals, but for households with children celebrating a birthday that number was nearly twice as high – an increase of 15.8.

In counties with low COVID-19 spread, however, the data did not show a jump in cases during the two weeks after birthdays, whether child or adult.

Workplace and business settings were under close scrutiny as policymakers tried to keep the public safe during the height of the pandemic, but statistics for informal gatherings have been difficult to come buy, according to the study’s lead author, Anupam Jena, a physician and associated professor of healthcare policy at Harvard Medical School. Birthdays, which were included in insurance claim data, gave “an opportunity to empirically quantify the potential role of small social gatherings in COVID-19 spread.”

“There’s a natural inclination to not think that your family members or friends are potentially infected or that you could potentially spread to family members or your friends,” Dr. Chris Whaley, an author of the study and policy researcher at the RAND Corporation, told USA Today.

The study also found that, when comparing counties with strict stay-at-home policies versus those without, there was no appreciable difference statistically. The same results were seen in counties that overwhelmingly voted for Trump versus those that didn’t.

“It certainly does suggest that people weren’t adhering to the shelter-in-place policies for this particular type of event,” Jena told The Guardian.

As of Monday, there were over 33,500,000 confirmed COVID-19 cases in the U.S. that led to 601,961 deaths, the most of any country in the world.